Field of the Invention
The invention relates to an auxiliary resonant commutated pole (ARCP) three-point or multipoint converter and, in particular, to soft-switching multipoint converter topologies for high-power converters.
Hard-switching multipoint converters, as have been proposed, for example, by A. Nabae et al. in the publication "A New Neutral Point Clamped PWM Inverter", in "Transactions of the IEEE Industrial Applications Society", Vol. 1A-17, No. 5, 1981, are used in the high-power field for controlling three-phase drives and, in power transmission systems, for gateways and compensation. The multipoint converter concept has been proven, in particular at high voltage levels for which the maximum reverse voltage of an individual active semiconductor device that is now available is inadequate.
At present, GTO switches with inverse diodes are used in multipoint converters with a voltage intermediate circuit in the high power field. In this configuration, the maximum current gradients di/dt and the voltage gradients du/dt that occur have to be limited by passive limiter networks, in order to avoid destruction of the active semiconductor devices. Such networks often have high losses, and contribute significantly to converter complexity and converter costs.
The maximum achievable switching frequency in these high-power converters is limited by the switching losses that occur in the semiconductor and by the minimum switching and recovery times of the semiconductor components. Since the switching frequency has a direct influence on the quality of the electrical input and output variables, and thus on the overall system configuration, the achievable switching frequency is a major quality criterion for a converter.
Progress in power-semiconductor development is now allowing converters to be operated with a considerably greater di/dt and du/dt, and this has resulted in the limiter networks becoming considerably smaller, or even being dispensed with. The present achievable switching frequency is thus now governed essentially only by the maximum permissible semiconductor losses.
Various soft-switching converter topologies that allow the switching losses to be reduced have been proposed in order to increase the maximum switching frequency for converters in the low and medium power ranges. In particular, the "Auxiliary Resonant Commutated Pole" (ARCP) principle for two-point converters, proposed in U.S. Pat. No. 5,047,913 by R. De Doncker et. al, is highly suitable for reducing switching losses. In such an ARCP converter, a snubber capacitor is connected electrically in parallel with each main switch.
Furthermore, an auxiliary circuit is proposed, which contains an auxiliary switch that is electrically connected in series with a resonant inductance, and which connects the neutral point of a DC intermediate-circuit capacitor to one output connection of the converter phase.
In addition to the drastic reduction in switching losses, the ARCP principle also allows the maximum du/dt and di/dt to be controlled which, apart from the opportunity to use critical semiconductor switches, also results in a reduction in the load on the end turns in three-phase motors.
Possible ways to extend the ARCP principle to three-point converters with neutral point clamp (NPC) diodes have been proposed by Cho et al. at the IEEE PESC Conference 1996, German Patent DE 195 36 470 by Dr A. Mertens and M. Bruckmann and by D. G. Rouaud et. al. in U.S. Pat. No. 5,684,688. In these solutions, the converter output is once again connected to at least one resonant inductance, which may be connected independently, via at least two bi-directional switches, to the two voltage neutral points of the two converter levels in the three-point converter. The difference in the topologies is the way in which the snubber capacitances for the four main switches are disposed. The number and configuration of the snubber capacitances have been varied considerably in an attempt to solve the problem of asymmetric charging movement between the upper and the lower converter level during commutation. However, it has not yet been possible to find the ideal situation, which guarantees maximum main switch load relief and a uniform capacitor load, that is to say a parallel circuit containing exactly equal snubber capacitances as close as possible to the respective main switch.